1. Field of the Invention
The present invention relates to a lens sheet used in a shadow mask for a display device such as a transmission screen for a projection TV, a Plasma Display Panel (PDP), a Liquid Crystal Display (LCD) and so on; and a producing method of the lens sheet. In particular, the invention relates to a method of forming a shielding layer with simple in a lens sheet having a fine pitch of 0.3 mm or less required for a pixel motor mode projection TV screen using a liquid crystal panel. Further, the invention relates to a method of forming a shielding layer in a fresnel lens which can remarkably improve contrast by reducing stray light.
2. Description of the Related Art
A transmission screen used in a projection TV is generally an assembly of a fresnel lens sheet and a lenticular lens sheet. In the projection TV adopting a CRT as a light engine (hereinafter will be referred to as CRT type projection TV), the lenticular lens sheet 11 is provided with convex cylindrical lenses 12 and 13 at both planes for radiation/incidence of image light as shown in FIG. 11. The lenticular lens sheet 11 functions to compensate the color difference in RGB 3-colors from the CRT as well as enlarge the angular field of image light incident to the lenticular lens sheet from the fresnel lens sheet.
Further, the lenticular lens sheet 11 has protrusions 14 each formed in boundaries of the convex cylindrical lenses 13 and a light-absorbing shielding layer 15 each formed on the tops of the protrusions 14. The light shielding layer 15, which is generally called black stripes, can prevent reflection of any external light from an observer as well as improve the contrast of image light radiated from the lenticular lens sheet 11.
The above described lenticular lens sheet 11 is formed by pressing a transparent thermoplastic resin sheet. Alternatively, the transparent thermoplastic resin sheet undergoes molding as well as extrusion (hereinafter will be referred to as extrusion molding) to form the convex cylindrical lenses 12 and 13 and the protrusions 14. The formed lenticular lens sheet is provided with the light shielding layer 15 by selectively printing light-absorbing colored pigment only on the protrusions 14 according to various printing methods such as screen printing and gravure printing.
In forming the above described lenticular lens sheet 11, location matching between in front and rear sides, which are the convex cylindrical lenses 12 of the incidence plane and the convex cylindrical lenses 13 and the shielding layer 15 of the radiation plane, is important. If axes of the both lenses in the front and rear sides do not have positional relation corresponding to the light incident to the convex lenses on the incidence plane, the shielding layer 15 blocks radiation light to degrade the lightness of radiation thereby deteriorating screen characteristics.
The lenticular lens sheet for the CRT type projection TV adopts the above described pressing or extrusion molding so that the location is performed in a relatively easy manner at the front and rear sides of the sheet. Also, the forming can be so performed to impart sufficient screen characteristics to the sheet. Alternatively, the lenticular lens sheet may adopt a microlens array sheet where microlenses are arranged. The microlens array sheet functions to widen the angular field of the projection TV not only horizontally but also vertically. Hereinafter in the application, the convex cylindrical lens, the microlens and the fresnel lens will be generally referred to as xe2x80x9clensxe2x80x9d and the lenticular lens sheet, the microlens array sheet and the fresnel lens sheet will be generally referred to as xe2x80x9clens sheet.xe2x80x9d
In order to obtain high quality and high definition of images, the projection TV requires to adopt an LCD projector, a Digital Micromirror Device (DMD) and so on having smaller pixels as a light engine (i.e. pixel type projection TV) rather than a CRT having larger pixels. However, when the related lens sheet for the CRT type projection TV is diverted into the pixel type projection TV, interference fringes (i.e. moirxc3xa9) are created as a drawback since the size of the pixels projected on the screen is similar to the pitch of lenses in the lens sheet. Therefore, in the lens sheet for the pixel type projection TV, the lenses are required to have a fine pitch, in particular at 0.3 mm or smaller, in order to prevent the moirxc3xa9 between the lenses and the pixels projected on the screen.
It is necessary to mold both sides of the lens sheet in the related method for forming a shielding portion by printing convex or concave regions in respect to the concave and convex pattern of the radiation plane in the lenticular lens sheet. However, in the pressing or extrusion molding, it is extremely difficult to form the fine-pitched lenticular lens sheet through the extrusion molding on both sides by the following reason. The fine pitch tends to degrade the molding rate of lens or incur axial deviation at the front and rear sides.
Accordingly, it has been often considered a technique for radiating light from the side of a related lens plane to activate a photosensitive layer applied to the opposite side in order to install a shielding layer only in a non-radiating portion which will not radiate light. Japanese Laid-Open Patent Application No. S59-121033 and H9-120101 disclose methods of blacking a tacky labyrinthine light portion by radiating light from the lens plane side to a positive photosensitive resin layer applied to the opposite side of the lens plane to cure the resin layer, and then printing powder colorant or black pigment or attaching a transfer film printed with black pigment to the photosensitive resin layer. However, those methods have problems that an additional process step is necessary to remove the colorant or pigment from the light condensing portion and a large amount of substrate film used as the transfer film heavily burdens the environment with waste materials. Further, it is difficult to completely remove the pigment or colorant from the light condensing portion. Also, Japanese Laid-Open Patent Application No. 2000-2802 discloses a method of providing a shielding layer by controlling hydrophilicity on the surface with photo-catalyst. However, since the light condensing portion of the lens becomes hydrophilic according to this method, the whole surface of the light condensing portion is printed after applying water. So, an additional process step is required to remove ink from a portion where water is applied thereby sophisticating the whole process.
In the meantime, as shown in FIG. 7A, a typical fresnel lens sheet 6 includes a flat incidence plane 7 of image light and a radiation plane 8 constituted of fresnel lens faces 9 and rising faces 10. As shown in FIG. 7B, image light 21 from a light engine 20 is incident on the fresnel lens sheet 6 as spreading. Therefore, the inside of the image light 21 is mostly radiated from the fresnel lens 9 to a lenticular lens sheet (not shown). However, as shown in FIG. 7C, the smallest portion inside the incident image light 21 reaches the rising faces 10 instead of fresnel lens faces 9. After reaching the rising faces 10, the image light directly radiates toward the lenticular lens sheet or reflects from the rising faces 10 before radiates from the fresnel lens faces 9. Such image light is called stray light since it may not be perpendicularly incident to the lenticular lens sheet. The stray light mixes noise into the regular image light to degrade the contrast of the projection TV. Known in the art is a technique for installing a light absorbing layer or a light scattering layer on the rising faces in order to restrain the stray light from occurring. (Refer to Japanese Laid-Open Patent Application No. S50-123448, Japanese Laid-Open Patent Application No. S52-143847, Japanese Laid-Open Patent Application No. S62-251701, Japanese Laid-Open Patent Application No. S62-251702 and so on.) However, it is difficult to install the light absorbing layer only on the rising faces, and even if properly installed, the manipulation is troublesome as drawbacks.
It is an object of the present invention to provide a lens sheet applicable to a screen for a pixel type projection TV and a simple producing method thereof, in which a shielding layer is formed without any axial deviation on front and rear sides of the sheet.
It is another object of the invention to provide a fresnel lens sheet and a simple producing method thereof, in which the fresnel lens sheet is attached with a shielding layer capable of remarkably enhancing the contrast of a projection TV by reducing stray light radiated from the fresnel lens sheet.
To achieve the above object, there is provided a lens sheet comprising a lens portion with a plurality of lens elements arranged in at least one side and a shielding layer provided in a non-transmitting portion of a light radiation plane, wherein the shielding layer is provided on a layer made of a cured photo-curing composition (A), wherein the photo-curing composition (A) is composed of 100 weight parts of photo-curing resin composition (a) having a surface free energy of 30 mN/m or more and 0.01 to 10 weight parts of compound (b) having a surface free energy of 25 mN/m or less.
In another aspect of the present invention, there is provided a lens sheet comprising a lens portion with a plurality of lens elements arranged in at least one side and a shielding layer provided in a non-transmitting portion of a light radiation plane, wherein the shielding layer is provided on a layer made of a cured photo-curing composition (A), wherein a light transmitting portion in the layer of the photo-curing composition (A) has a surface free energy of 25 mN/m or less.
In yet another aspect of the present invention, there is provided a method of producing a lens sheet which includes a lens portion with a plurality of lens elements arranged in at least one side and a shielding layer provided in a non-transmitting portion of a light radiation plane, the method comprising the following steps of: coating photo-curing composition (A) on a light radiation plane of the lens sheet to form a layer made of the photo-curing composition (A), the photo-curing composition (A) being composed of 100 weight parts of photo-curing resin composition (a) having a surface free energy of 30 mN/m or more and 0.01 to 10 weight parts of compound (b) having a surface free energy of 25 mN/m or less; radiating light from the side opposite to the layer of the photo-curing composition (A) to selectively cure a light transmitting portion of the layer of the photo-curing composition (A) with the layer of the photo-curing composition (A) being contacted with a medium having surface free energy lower than that of the compound (b); and painting colored pigment on the layer of the photo-curing composition (A) to form a shielding layer in a non-transmitting portion of light.
The method of the invention further comprising the step of: radiating light from the side opposite to the layer of the photo-curing composition (A) to cure an uncured portion of the layer of the photo-curing composition (A) with the layer of the photo-curing composition (A) being contacted with a medium having surface free energy higher than that of the photo-curing resin composition (a) between said step of radiating light from the side opposite to the layer of the photo-curing composition (A) to selectively cure a light-transmitting portion of the layer of the photo-curing composition (A) and said step of forming a shielding layer.
Herein, the value of the surface free energy in the specification is the value at temperature 20xc2x0 C., relative humidity 50 percent, and measured by the method described below.
There are various methods for measuring the surface free energy of a liquid, but in this specification, the values measured by the Wilhelmy method at a temperature of 20xc2x0 C. and a relative humidity of 50 percent were used as the surface free energy. The measuring principle of the Wilhelmy method is as follows. As shown in FIG. 5, a plate 52 is suspended from one arm of a balance 51, one end is immersed in a liquid 53 to be measured, a suitable load 54 is attached to the other end and the balance 51 is adjusted. In addition to gravity and buoyancy, the plate 52 is also subject to a force in the downward direction from the measurement liquid 53, so the following equation (1) may be written for the equilibrium state:
Load 54=Weight of plate 52xe2x88x92Buoyancy of plate 52+Force from liquid surfacexe2x80x83xe2x80x83(1)
As the force due to the liquid surface is equivalent to the surface tension (=surface free energy of the liquid), the surface free energy of the liquid can be obtained by measuring the force due to the liquid surface. The material of the plate 52 may be platinum or glass, etc., and as the surface free energy does not change, it may be a material which is not corroded by the measurement liquid 53. In the measurements according to the specification, platinum is used as the material of the plate 52.
The surface free energy of the solid cannot be measured directly, but the surface free energy can be found using various types of liquid known in the art. A liquid drop on a solid surface has the cross-sectional shape shown in FIG. 6. In the figure, a contact angle is an angle subtended by a tangent to the liquid 62 and the side of the surface of the solid 61 in contact with the liquid 62, at an intersection point between the surface of the liquid 62 on the surface of the solid 61, and the surface of the solid 61. Let this angle be ?. The following equation (2) (Young""s equation) is then satisfied due to the equilibrium conditions at the intersection point:
xcex3S=xcex3SL+xcex3Lxc2x7COS ?xe2x80x83xe2x80x83(2)
Here, xcex3S is the surface free energy of the solid, xcex3L is the surface free energy of the liquid, and xcex3SL is the surface free energy of the solid/liquid interface.
The surface free energy xcex3 is represented by the sum of the dispersion force component ?a, polar force component xcex3b and hydrogen bond force component xcex3c, so the following assumption (3) can be made regarding the surface free energy of the solid/liquid interface xcex3SL:
xcex3SL=xcex3S+xcex3Lxe2x88x922(xcex3Saxcex3La)1/2xe2x88x922(xcex3Sbxcex3Lb)1/2xe2x88x922(xcex3Scxcex3Lc)xe2x80x83xe2x80x83(3)
Therefore, if the value ? of the contact angle is found for three or more liquids having different free surface energies under the conditions of temperature 20xc2x0 C. and relative humidity 50%, xcex3Sa, xcex3Sb, xcex3Sc can be deduced by solving the equation with three unknowns derived from equation (2) and equation (3), and the surface free energy of the solid xcex3S can be found as their sum.